In recent years, safety to humans including, for example, the air pollution problem attracts world-wide attentions. In this connection, electric and electronic appliances are required to be more harmless and more safe in addition to the requirement of a high flame retardancy. To be more specific, the electric and electronic appliances are required to be resistant to flame and, at the same time, not to generate noxious gases. It has been customary in the past to use a glass/epoxy as a substrate of a printed wiring board on which electric and electronic appliances are to be mounted. In general, a brominated epoxy resin containing bromine as a flame-retardant, particularly, tetrabromobisphenol-A type epoxy resin, is used for forming the substrate of the printed wiring board.
The brominated epoxy resin certainly exhibits a high flame retardancy, but generates a noxious hydrogen halide (hydrogen bromide) gas when burned. To overcome the difficulty, epoxy resin compositions containing non-halogen flame-retardants such as nitrogen compounds, phosphorus compounds, inorganic compounds, etc., have been developed, as reported in, for example, British Patent No. 1,112,139 and Japanese Patent Disclosure (Kokai) No. 2-269730. However, these flame-retardants give rise to a detrimental effect to the curing of the epoxy resin and impairs the humidity resistance of the cured composition. In addition, a glass cloth is less likely to be impregnated with the epoxy resin composition if the composition contains the non-halogen flame-retardants mentioned.
The present invention is intended to provide an epoxy resin composition which does not contain a halogen element but exhibits a good flame retardancy and which permits overcoming the above-noted problems inherent in the prior art.
The present invention is also intended to provide a prepreg impregnated with such an epoxy resin composition as well as a laminate, copper-clad laminate and printed wiring board using such a prepreg sheet.